Offline docs (Linux/macOS)

[rodlie]

Added offline docs for Linux and macOS.

• Selecting 'Documentation' from 'Help' will now check if offline docs exists and use that, else fallback to online docs.
• macOS DMG has a shortcut to documentation

The HTML is very basic with dark/light mode support, if someone want's to modify/improve it see:

• https://github.com/friction2d/friction2d.github.io/blob/main/assets/style-doc.css
• https://github.com/friction2d/friction2d.github.io/tree/main/_layouts_app
• https://github.com/friction2d/friction2d.github.io/blob/main/build_app_docs.sh

Windows support will be added later, as my Windows laptop needs a reinstall.